15 Aug 2008

Between catching the flu and awaiting an answer on strange results I’m getting from the Mitchell suspension software, I’m starting on the CAD drawings – may as well do something productive. Doing the drawings is already proving educational, forcing the need to address tubes that aren’t even on the mockup yet. One thing to decide is how to pass the coolant lines, shifter and throttle cables, brake and clutch lines, and wires from the front of the car to the back. Some people have suggested running the coolant lines outboard, but that doesn’t really simplify anything. They still need to be covered to avoid burns or having boiling coolant spraying about in case of a cracked line or accident. Even if they were run there, the throttle and clutch lines lend themselves best to running down the center of the car anyway, so may as well run everything that way.

14 Aug 2008

Today the decision was made on where and how the rear shocks will mount.

There’s going to be rocker-arms at the rear. Not because it’s ubersexy, just that it’s the most appropriate solution. Why? The Miata rear upright has its upper suspension pickup cushioned in a big rubber bushing. Putting a screwdriver through the bushing and pulling it from side to side shows that it’s fairly soft. I was going to mount the bottom of the shock to it, but applying an offset load of ~400 lbs to it isn’t a good idea. A solid bushing could be substituted for the rubber, but one design goal is to avoid lathe-work. On top of that, the steeply-inclined shocks gave too low an installation ratio, requiring spring rates higher than what’s commonly available in 10-12″ springs. They were also too close to the engine and exhaust for comfort, and that’s on my drivetrain. Who knows how close they’d be to whatever drivetrains builders are going to install. Tilting the shocks to be more upright wasn’t an option, either, because they’d stick through the engine cover with nothing to attach to. I would have attached the shocks directly to the lower suspension pickup point on the upright, but they’d would have to hang off a bolt in single-sheer about an inch out… won’t go there.

The best solution appears to be to keep the shocks away from the drivetrain and keep them low. While putting them behind the drivetrain makes them accessible, the exhaust is back there, too, cooking anything close by. For these reasons they’re being placed vertically, about a foot forward of axle centerline.

Speaking of the rubber bushing, if a single rod-end is bolted to one end, it’ll twist the bushing. If a U-shaped bracket is used so a bolt passes all the way through the assembly (like a Miata) then it’ll both bind slightly when toe is adjusted, and move “some amount” during cornering. I’ll probably leave it as-is until I figure out how much of an issue it is.

13 Aug 2008

With the front end sorted and the axles having arrived, attention returned to the rear suspension. Push-rod rear suspension was almost considered a requirement, but after spending hours staring at it and trying different solutions, an old-school shock layout was found. However, it’s very tempting to modify the Miata upright to make it work even better, but that has to be thought about a bit more.

It’s not easy routing the chassis around the engine, and while a de Dion axle layout makes things simpler, I don’t want to settle for almost-great, may as well do the best I can. A true independent suspension is also the most flexible, for those builders who want to tinker with things – or just leave it as-is.

One thing I’ve been ignoring is how to remove the drivetrain; haven’t decided if it’s coming out the top or the bottom. I kind of like the bottom-exit solution because the drivetrain doesn’t have to be lifted in and out, scratching up chassis tubes. Just wheel something under the car for the engine to sit on and lift the car up off it. Either way it means some of the tubes need to be removable, just haven’t decided which ones yet.

I have two more days to get everything sorted (before the wife comes home) and I think it’s doable, at least by the end of the weekend. Well, “sorted” in the sense that tube placement has been decided, then it’ll take weeks to put it all into CAD. I’m concentrating on only one side of the mockup; no point doubling the work of making the other half.

I realized that it’s impossible to go straight to steel without completely recording every dimension. Since I don’t have space to set the mockup aside once steel construction starts, it has to be chopped up for firewood before I can. If I missed some critical dimension, well, that would be unfortunate. For that reason it’ll go to CAD first. Now that I’m nearly out of the hardest part of the project, I’m starting to change my mind on how to do mockups. That is, I now think it would be easier and maybe even faster to do it all in CAD (“next time”) though detailed dimensions are still needed of all major components.

While taking Cooper for his morning walk, we had another encounter with an animal. Much like the ferret from a few years back, we were walking along a trail and out of a bush came a light tan and white rabbit. It must have been someone’s escaped pet, due to the coloration and also because he walked right up to us. I thought of picking him up, but then what? Cooper, who thinks rabbits taste wonderful, had never had one actually walk right up to him – he didn’t know what to do. When he finally decided that it might really be a rabbit, it took off when he barked at it. Hope the little fellow finds his way home; being in the wild with white markings is like having a bulls-eye on your head.